All references cited in this specification, and their references, are incorporated by reference herein in their entirety where appropriate for teachings of additional or alternative details, features, and/or technical background.
Flexible imaging belts include electrophotographic imaging belts, ionographic/electrographic imaging belts, and intermediate image transfer belts for transferring toner images used in an electrophotographic or an electrographic imaging system. Such flexible imaging belts may include photoreceptor layers containing a substrate, an electrically conductive layer, an optional hole blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer and, in some embodiments, an anti-curl backing layer. A layered photoreceptor having separate charge generating (i.e. photogenerating) and charge transport layers is described in U.S. Pat. No. 4,265,990.
Flexible imaging belts may be fabricated from a cut sheet of an imaging member web. The sheets which may comprise square, rectangular or parallelogram shapes can be configured into a belt by joining the overlapping opposite marginal ends of the sheet to form a seam. The joining technology may involve welding (including ultrasonic welding), gluing, taping, or pressure heat fusing.
Ultrasonic welding may be the method chosen for joining a flexible imaging member because it is rapid, clean and solvent-free and low cost, as well as because it produces a thin and narrow seam. In addition, ultrasonic welding may be preferred because the mechanical high frequency pounding of the welding horn causes generation of heat at the contiguous overlapping end marginal regions of the flexible imaging sheet loop to maximize melting of one or more layers therein to form a strong and precisely defined seam joint.
Ultrasonic welding is a process that uses high frequency mechanical vibrations above the audible range. The vibrations are produced at the tip of a welding sonotrode or horn. The vibratory force emanating from such a horn device can be generated at high enough frequencies to soften or melt thermoplastic material components intended to be joined together. For example, such frequencies can be effective at 20, 30 or 40 kHz. One of the main advantages of ultrasonic welding may be found in the very short welding steps that enhance its usefulness even in mass production. Weld times may last less than a second. Thus, the process has been utilized in many industries and applications.
Ultrasonic welding can be accomplished at various distances from the horn ranging from only a fraction of a millimeter up to several centimeters. For distant welding the polymer must transmit the energy efficiently, i.e. not be too flexible or have too high a loss modulus. A copolymer of acrylonitrile, butadiene, and styrene (ABS) and high impact polystyrene is among the easiest polymers to weld ultrasonically. Ultrasonic welding will usually join amorphous thermoplastics more readily than semicrystalline ones. However, the advent of more powerful machines has blurred this distinction, and semicrystalline polymers are now welded routinely.
The ultrasonic welding process may entail holding down the overlapped ends of a flexible imaging member sheet with vacuum against a flat anvil surface and guiding the tip end of an ultrasonic vibrating horn transversely across the entire width of the sheet, over and along the overlapped ends, to form a welded seam. The ultrasonic vibration frequency applied for joining the photoreceptor belt/loop ends is kept so high that a frictional heat results upon contact with material to be joined. The heat causes softening or melting of contact portion which results in fusing the joined belt end pieces without any horn burn blemishes in the form of undesirable raised, rough and brittle welds.
Ultrasonic welding is probably the most commonly used thermoplastic welding process because it is very fast (fractions of a second to a few seconds) and usually produces welds that are relatively free of flash. In addition, ultrasonic welding can be automated and thus is particularly suitable for high volume production.
Rapid development of the ultrasonic welding machine has occurred in the last ten years. Basic functions, such as weld energy, collapse, trigger force, and pressure are now microprocessor controlled. In addition, real time feedback and control of welding conditions is being offered, along with the ability to vary weld force and amplitude during the weld cycles.
Welding by ultrasonic devices requires a tool design suitable for the particular task at hand. The mechanical characteristics of the substrate will determine the selection of the welding machine. An ultrasonic welding device typically includes four main components, a power supply, a converter, an amplitude controlling device or booster, and an acoustic tool which is called the horn or sonotrode. The electricity is changed by the power supply, for example, from 50–60 Hz into a high frequency such as 20, 30, or 40 kHz and then supplied to a converter. The converter may comprise discs of piezoelectric crystals wedged between two metal sections and kept tightly compressed to respond to even the slightest pressure change. The converter serves to change the electrical energy into mechanical vibration energy at high ultrasonic frequencies. The vibratory energy is transmitted through the booster. The booster increases the amplitude of the sound wave to the horn.
The horn is an acoustical tool delivering ultrasonic vibratory energy directly to the substrate portions being assembled. In addition, the horn is used to apply welding pressure. The vibrations are transmitted from the horn to the joint area or seam, where the resultant friction causes the surface of the substrate material to soften or melt, and subsequently fuse together. Ultrasonic welding may be used to join flexible image photoreceptor loops as well as intermediate transfer belts and weldable polymer substrate belts.
Flexible image loops photoreceptors and imaging belts may be joined together at opposite ends to form a continuous loop by ultrasonic acoustic welding horns which transfer vibration energy to the bondable substrates. The ultrasonic welding process involves flexing of the slender projection or tip portion of a metallic welding horn member by oscillating at rates of 10,000 to 70,000 times per second (kHz). The oscillation causes the horn tip portion to move across the flexible belt joint area to create a weld or seam. The horn member can be held at a distance from the joint surface suitable for effective mechanical energy delivery to the joint area which rests on its opposite side on, for example, on an anvil ensuring that most of the energy is spent in the weld zone of the joint area. The overlapping ends of the flexible thermoplastic belt sheet consequently melt forming a weld or seam. Frictional heating can also occur to some extent because transmission of the energy through the plastic parts is very complex.
An oscillating force of an ultrasonic horn is generated when alternating electrical power (at frequency) is applied to a train of tuned components that are sized to form a resonant system. The first component converts the electrical power (i.e. voltage) to oscillations. This occurs when the power is applied to a sandwich of piezoelectric or magnetostrictive materials and metal blocks. These oscillations are amplified (or de-amplified) by a booster and the booster is connected to the horn. The horn can either amplify or de-amplify the oscillations, depending on the needs of the application. While the frequency of oscillations vary between 10 and 70 kHz, the most common frequencies are in the range from 20 to 40 kHz. Oscillation amplitudes range from 20 to 80 microns.
The piezoceramic material may include one or more of barium titanate (BaTiO3), lead zirconate titanate (PZT), or lead titanate (PbTiO3). Most preferably, the piezoceramic is PZT. The polymer of the composite may include any suitable binder polymer, and may or may not itself be piezoelectric. Piezoelectric polymers include polyvinylidene fluoride (PVDF), and copolymers of vinylidene fluoride and trifluoroethylene (PVDF/TrFe) or vinylidene fluoride and tetrafluoroethylene (PVDF/TeFe). Other binder polymers may include, for example, epoxies, silicone resins, cyanoacrylates, etc., without restriction. A preferred polymer is an epoxy in that it can also act to strongly adhere the piezoelectric composite to the platform section of the horn member.
The 1–3 configuration nomenclature that identifies the configuration of the piezoelectric composite transducer is known in the art and refers to the two-phase piezoelectric behavior of the material, the first number referencing the physical connectivity of the active phase (z direction) and the second number referencing the physical connectivity of the passive phase (y direction). The 1–3 composite configurations have been found to be most advantageous in achieving consistently uniform tip vibration amplitude. For example, the piezoelectric composite may be bonded with an adhesive layer to horn. A vast array of adhesives such as transfer adhesives, epoxies, cyanoacrylates, or an epoxy/conductive mesh (e.g., metal screen) layer may be used to bond the horn and piezoelectric element together.
Ultrasonic welding may lead to a welding defect known as “Horn Burn.” “Horn Burn” results in a raised, rough, and brittle welds, which are all unwanted traits. “Horn Burn” has been associated with the ability of the horn to transfer heat out of the seamed area. While numerous materials have been used in the fabrication of horns, aluminum has shown itself to be an effective head dissipating material. The main disadvantage of aluminum is its inherent softness. The configured ultrasonic horn tip wears away after several weld cycles. The constant wear due to the welding friction results in equipment downtime for converting, the cost of ultrasonic horn tooling, and defective belts. Various ceramic plating methods have been tried in the past to improve the wear resistance of ultrasonic horns, but have shown a tendency to delaminate. In respect of ceramic plating, it has been found that the ultrasonic energy's elongation of the welding horn may break the plating bond after only a few cycles.